Field of the invention
The invention relates to a method for producing purified and concentrated MgCl.sub.2 -brine by evaporation and crystallization from brines containing MgCl.sub.2, KCl, NaCl and MgSO.sub.4.
Description of the prior art
Different methods for purification and concentration of magnesium containing brines are known from the patent literature. Large quantities of such brines are obtained as a by-product from the production of potassium chloride, potassium sulphate and sodium chloride. Such brines can be purified and concentrated in different ways in order to obtain a highly concentrated MgCl.sub.2 -brine, with a low content of KCl, NaCl and MgSO.sub.4, which is suitable as a raw material for the production of magnesium metal or magnesium oxide.
German Pat. No. 676406 shows how solutions with less than 320 g/l magnesium chloride can be evaporated until saturation with carnallite is reached. The sodium chloride-kieserite mixture which crystallizes due to evaporation is then separated, the hot brine is cooled and the carnallite which crystallizes due to cooling is separated from the product brine. However, the solutions obtained in this manner still contain about 1% potassium- and sodium chloride and considerable quantities of magnesium sulphate. The sulphate ions have to be removed by precipitation with lime. After further evaporation additional mixtures of carnallite and kieserite have to be separated from the brine at temperatures up to 130.degree. C., before a concentrated brine of sufficient purity is obtained by cooling and separation of a crystalline carnallite/sodium chloride mixture.
In German Patent No. 1667826 it is proposed to concentrate magnesium chloride brines containing sulphates by evaporation under vacuum at temperatures of 60.degree.-90.degree. C., and subsequent heating of the solution at atmospheric or slightly elevated pressure to 108.degree.-130.degree. C., followed by isothermal removal of the sulphate compound and cooling of the solution obtained to 25.degree. C. The solution is then concentrated to 50 weight % magnesium chloride hexahydrate by vacuum evaporation at 60.degree.-90.degree. C. After cooling crystalline Bischofite is obtained.
From German Offenlegungsschrift No. 2613288 it is known that a highly concentrated magnesium chloride solution can be obtained after debromination with chlorine, sulphate precipitation with calcium chloride, increasing the magnesium chloride content to 270-330 g/l by carnallite decomposition, and subsequent concentration by evaporation and cooling to ambient temperature, resulting in the crystallization of carnallite. The solution which is obtained after removal of the crystals is the required concentrated brine product.
According to German Pat. No. 2613289 a highly concentrated magnesium chloride solution can be obtained from dilute solutions by evaporation to 440-475 g/l magnesium chloride, separation of carnallite and sodium chloride, debromination with chlorine, subsequent precipitation of sulphates by means of a lime- or dolomite suspension, separation of the precipitate, followed by crystallization of a mixture of carnallite/sodium chloride by cooling. It is claimed that the formation of sulphate containing double salts, which would lead to loss of potassium values, is prevented by this method.
In German Patent No. 2513947 on the other hand the formation of sulphate containing langbeinite is desired. After evaporation of the solution the crystallization of langbeinite is promoted by nucleation by addition of langbeinite crystals. After further evaporation and subsequent cooling crystallization carnallite is separated from the product brine and contacted with the crude brine for decomposition. The sulphate salts are converted to potassium sulphate.